I have tried both the disk utility and the fsck procedure. Neither of them solved the issue.

Would an archive and install be likely to fix this problem? I just learned that if I don't leave my machine on 24hrs a day, I might have problems as well? Could that be the cause (can be cured by something called MacJanitor?)?

I'm really new to this kind of problem as I've had my Mac for almost two years with no problems. I'm far more used to solving PC problems (as they are so frequent).